The microminiaturization of an ion beam is required for enhancing performances in the fields of the dry microprocess such as for example ion beam lithography, dry development, and micro-doping, and submicron surface analysis such as, for example three-dimensional analysis including also the depth direction), etc. therefore considerable activity has been devoted to developing a point ion-source of high brightness.
To the end of the microminiaturization of an ion beam, it is desirable to develop an ion source which has to properties of high brightness, small effective source-size, high angular intensity, and narrow energy width. An ion source which nearly satisfies the noted properties, is an electrohydrodynamic or "EHD" ion source.
An EHD ion source of the aforementioned type is described in detail in U.S. Pat. No. 4,088,919. The fundamental principle of the EHD ion source is based on the phenomenon that, when an intense electric field of 10.sup.6 -10.sup.8 V/cm is applied to a pointed end of an electrode formed as of a pipe having an inside diameter of approximately 100 .mu.m filled a liquefied metal or a conductive liquid or a needle shaped electrode having a pointed end a radius of curvature of below several .mu.m, wetted with a liquefied metal, ions of the liquid component are emitted therefrom. The mechanism of the ionization is not yet fully understood.
FIG. 1 shows the fundamental construction of a prior-art EHD ion source of the needle type and according to this figure, an electrode generally designated by the reference numeral 10 provided with a tip 2 having a pointed end with a radius of curvature of below approximately 10 .mu.m, is spot-welded to the central part of a hairpin-shaped filament 1. A central part 8 of the filament 1 carries a liquefied metal 3, for example, Ga. A high voltage V.sub.1 is applied between an extractor 4, provided with an aperture 9 and disposed below the tip 2 and the electrode 10, by an extracting power supply 6 so as to give the extractor 4 a negative potential and to establish an electric field of 10.sup.6 -10.sup.8 V/cm at the pointed end of the tip 2. Then, ions 5 of the component of the liquefied metal 3 are emitted from the pointed end of the tip 2 wetted with the liquefied metal 3. This is the operating principle of the EHD ion source. A voltage V.sub.o applied across both the ends of the filament 1 is a voltage for heating the filament 1 in order to keep the liquefied metal 3 in the liquefied state, and it is supplied by a heating power supply 7.
FIGS. 2A-2D illustrate the manner in which the surface profile of the liquefied metal 3 carried on the central part 8 of the electrode 10 varies depending upon the magnitude of the extracting voltage V.sub.1. More particularly, as shown in FIG. 2A, the liquefied metal 3 is not carried by the electrode 10; however, in FIG. 2B the liquefied metal 3 is carried by the electrode 10, but the extracting voltage V.sub.1 is null. As apparent from FIG. 2B, when the extracting voltage V.sub.1 is null, the surface profile of the liquefied metal 3 extends substantially along the shape of the electrode 10. When the extracting voltage V.sub.1 is gradually increased to 10 kV, the surface profile of the liquefied metal 3 is altered to the profile shown in FIG. 2C. As shown in FIG. 2C, under the action of the electric field, the surface profile of the liquefied metal 3 is somewhat expanded from the shape of the electrode 10. When the extracting voltage V.sub.1 is further increased to 13.5 kV, the surface profile of the liquefied metal 3 assumes the profile shown in FIG. 2D, wherein is greatly expanded from the shape of the electrode 10. In an experiment, when the extracting voltage V.sub.1 was increased to 14 kV, the liquefied metal 3 could not endure the action of the great electric field and for the most part, dropped from the electrode 10. A experiment was conducted by employing a flat electrode as the extractor 4 and setting the distance between the pointed end of the tip 2 and the extractor 4 at 10 mm.
FIG. 3 is a graph showing the relationship in the above experiment between the extracting voltage V.sub.1 and the ion current I.sub.T obtained at that time. The ion current I.sub.T was measured with the extractor having no aperture 9 and by means of an ammeter disposed between the extractor 4 and ground. As apparent from FIG. 3, the electric field of the pointed end of the tip 2 increases with the increase of the extracting voltage V.sub.1. At a point in time when a certain threshold value V.sub.t1 (approximately 6.4 kV) is exceeded, the ion beam 5 of the liquefied metal 3 begins to be emitted from the pointed end of the tip 2. The electric field is established to be the most intense at the pointed end of the tip 2. Since, however, the elecric field is formed also in the other surface parts of the liquefied metal 3, the liquefied metal 3 itself is drawn in the direction of the electric field. When the field intensity is too high, not only is the liquid profile of the liquefied metal 3 changed from the previous conical shape into the flat shape as shown in FIG. 2D, but also the quantity of supply of the liquefied metal 3 towards the pointed end of the tip 2 becomes large. Regarding the quantity of the liquefied metal 3 at the pointed end of the tip 2, it is ideal that the quantity to be emitted as the ions 5 balances with the quantity to be supplied from the root part of the tip 2 to the pointed end thereof. If the quantity supplied to the pointed end of the tip 2 is larger than the quantity emitted in the form of the ions 5 from the pointed end of the tip 2, the quantity of the liquefied metal 3 at the pointed end of the tip 2 becomes excessive. Therefore, the radius of curvature of the pointed end of the tip 2 becomes large, and the intensity of the electric field established at the pointed end of the tip 2 lowers. As a result, as seen from the graph of FIG. 3, while the extracting voltage V.sub.1 is in a low voltage range, the ion current I.sub.T tends to increase with the increase of the extracting voltage V.sub.1, whereas, when the extracting voltage V.sub.1 exceeds a certain value, the ion current I.sub.T tends to abruptly decrease in spite of the increase of the extracting voltage V.sub.1.
That is, with the construction of the prior-art EHD ion source shown in FIG. 1, the control of the magnitude of the ion current I.sub.T is made by the increase or decrease of the extracting voltage V.sub.1. Therefore, when it is intended to obtain a great ion current I.sub.T by applying a great extracting voltage V.sub.1, the electric field rather weakens due to the change of the shape of the pointed end of the tip 2, so that even when a voltage in excess of a certain specific value is applied a greater ion current cannot be generated. This leads to the problem that there is the limitation to the magnitude of the ion current I.sub.T which can be derived.